Al Jazeera English - News - Us Military Chief Warns Tehran

"But ... please do not misread restraint for lack of resolve, and those ship COs [commanding officers] will defend themselves."

Warning shots

Meanwhile, a Pentagon official said that similar incidents had happened in the Strait of Hormuz on two other occasions in December, including one in which a US warship had fired warning shots.

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The USS Whidbey Island, an amphibious warfare ship, fired the warning shots after a single Iranian speedboat approached it at high speed on December 19, the official, who asked not to be identified, said.

The second incident involved the USS Carr, a guided missile frigate, that "came in very close contact with three small boats" as it transited the Strait on its way into the Gulf, the official said.

But the chairman of the Joint Chiefs of Staff insisted that the January 6 incident was the most serious yet.

"There have been other situations where certainly ships transiting the Straits of Hormuz have been approached," Mullen said.

"To my knowledge, I've not seen one as both provocative and dramatic as this."

'Aggressive' manouevres

According to the United States, five Iranian speedboats manoeuvered aggressively close to three US ships, which also received a radio transmission threatening an explosion.

The 36-minute version aired on Friday included the footage of that incident, which was in the earlier version, plus extended and largely uneventful footage of Iranian boats following the US ships at some distance.

Iran, which has described the encouter as routine and ordinary, has aired its own video showing an Iranian commander in a speedboat contacting an American sailor via radio, asking him to identify the US vessels and state their purpose.

Pentagon officials have acknowledged that they cannot confirm the origin of the that the radio broadcast in which a voice is heard to say in accented English: "I am coming to you ... You will explode in a few minutes".